Abstract

Background

We have recently shown that undomesticated strains of Bacillus subtilis can extensively colonize the surfaces of rich, semi-solid media, by a flagellum-independent
mechanism and suggested that sliding motility is responsible for surface migration.
Here we have used a flagella-less hag null mutant to examine and confirm sliding motility.

Results

Using a defined semi-solid medium we determined that a B. subtilis hag mutant colonized the surface in two stages, first as tendril-like clusters of cells
followed by a profuse pellicle-like film. We determined the levels of macro- and micro-nutrients
required for the tendril-to-film transition. Sufficient levels of each of the macronutrients,
glycerol, Na-glutamate, and Na-phosphate, and inorganic nutrients, K+, Mg2+, Fe2+ and Mn2+, were required for robust film formation. The K+ requirement was quantified in more detail, and the thresholds for complete tendril
coverage (50 μM KCl) or film coverage (2–3 mM KCl) were determined. In addition, disruption
of the genes for the higher affinity K+ transporter (KtrAB), but not the lower affinity K+ transporter (KtrCD), strongly inhibited the formation of both tendrils and films,
and could be partially overcome by high levels of KCl. Examination of hag tendrils by confocal scanning laser microscopy revealed that tendrils are multicellular
structures, but that the cells are not as highly organized as cells in wild-type B. subtilis pellicles.

Conclusion

These results suggest that B. subtilis can use sliding motility to colonize surfaces, using a tendril-like growth mode when
various macronutrients or micronutrients are limiting. If nutrients are balanced and
sufficient, the surfaces between tendrils can be colonized by robust surface films.
Sliding motility may represent a strategy for nutrient-deprived cells to colonize
surfaces in natural environments, such as plant roots, and the media described here
may be useful in investigations of this growth phenotype.